1. Field of the Invention
This invention relates to an exposure control method in an automatic color printer, and more particularly to a method of controlling exposure in an automatic color printer which adopts a gray integration printing method wherein slope control is improved.
2. Description of the Prior Art
In an automatic color printer in which the exposure is controlled in accordance with a gray integration printing method based on LATD (Large Area Transmittance Density), the density and color balance of a printed photograph change as the density of the original negative changes. In other words, even if the same subject is photographed, the density and the color balance of the photograph printed by the printer change as the density of the original negative changes. This is considered to be caused by the reciprocity law failure of the photographic paper, the difference between the spectroscopic sensitivity of the photographic paper and that of the printer, and the reciprocity law failure of the electric circuit employed in the printer and so forth.
In order to correct the change in density and color balance as mentioned above, a slope control is performed. The slope control is a process employed in the color printer by which the slope of a curve in a graph having a scale of LATD on the abscissa and exposure time on the ordinate is changed of its gradient in order to obtain a color print of constant density and color balance with negatives of different density.
It has been known in the art to perform the slope control in an automatic color printer. In the conventional color printer with the slope control system, the exposure time is corrected with reference to the density of the negative by use of a capacitor connected in series with a photodetector which measures the density of the negative. The exposure time is elongated when the negative is of over exposure, and is shortened when it is of under exposure.
The above-described conventional color printer with the slope control system has a defect in that the linearity of the slope control (the relationship between the density of the negative and the log of the exposure time) is low since the slope control system utilizes the characteristic of charging and discharging of a capacitor. Further, since the slope control system is combined with an exposure control circuit in the conventional color printer, there is also a defect in that the slope control system and the exposure control circuit affect each other.